1. Field of the Invention
This invention relates to a low pressure axial flow steam turbines, and in particular, to a scheme for removing moisture carried by the steam flow within a low pressure turbine apparatus.
2. Description of the Prior Art
As is well known to those skilled in the art of steam power generation, one of the major problems associated with low pressure wet steam turbine utilized in nuclear power plants is the control of the increasing amount of moisture created as the motive steam expands through the turbine apparatus. Since the presence of excessive moisture carried by the axial flow increases erosion damage to rotating blades and also reduces the thermal efficiency of the apparatus, it is highly desirable to remove condensed steam droplets carried by the steam flow or else to minimize their effects.
In order to inhibit damage to the rotating blades, it is the practice in the prior art to dispose a coating or a strip of hard material, such as Stellite, on the backside of each of the rotating blades usually in the last and next to last rotating blade arrays. It is these rotating blades which undergo the most severe erosion damage since, at this point in the turbine, the moisture carried by the steam has agglomerated to a drop size of sufficient mass that it cannot be accelerated through the small axial clearance between the stationary and rotating blades and thus impinges upon the backside of the rotating blades. The incessant bombardment of the backside of rotating blades by these relatively massive water droplets severely damages the blades. Erosion damage of such severity has been documented that the damaged air foil is nearly severed from the remainder of the blade adjacent the blade tip. However, provision of erosion resistant material, such as Stellite, is expensive, and often tends to weaken the rotating blades.
An alternative method for controlling the effects of condensate by modification of the rotating blades is shown in U.S. Pat. No. 3,290,004, issued to Ishibashi, which provides radially extending grooves on the backside of the rotating blades. Also, a similar rotating blade has been disclosed in General Electric Company 1969 Presentation to the American Edison Illuminating Companies.
Other moisture removal schemes have heated the stationary blades to prevent the agglomeration thereon of larger more massive water drops. By providing a heated outer coating on the stationary blades, the fine fog droplets carried by the steam flow are discouraged from being deposited on the stationary blades, thus drops of the size sufficient to damage the rotating blades are not able to form. Another technique is disclosed and claimed in U.S. Pat. No. 3,724,967, issued to Frederick K. Fischer and assigned to the assignee of the present invention. Fischer disposes interconnecting cavities in the base portion of the stationary blades which communicates with ports on the surface of the blade and which removes steam and water from the surface of the stationary blades separate the steam and water phases, and direct the steam toward the rotating blades downstream thereof and water to an appropriate drain.
In addition to these above-mentioned internal moisture-removal devices, the prior art has disclosed moisture separation elements external to the turbine. For example, an external moisture-separator element is often disposed in the crossover line between the high and low pressure turbine elements in nuclear power plants. As another example, steam is often reheated after it has been partially expanded through the high pressure element by adding thereto heat taken from throttle steam. However, these external methods are often cumbersome and require significant amounts of external piping which, adds to the cost and the space requirements of the power plant.